長野市南部，後期更新世高野層における元素組成からみた珪藻生産量の変動とその古陸水学的意味

摘要

長野市信東町に分布する高野層のボーリングコア（TKN－2004）について，14～4万年前までの化学組成に基づく珪藻生産量の変動を検討した．高野層のSiO_2／Al_2O_3比は，生物源シリカ量と相関を持つので，珪藻生産量の指標となる．珪藻生産量と花粉に基づく環境指標を比較した結果，珪藻生産量はおもに気温に支配された可能性が高い．珪藻生産は，深水層の栄養塩が有光層まで回帰する時に高まることから，気温が表層水温を規制し，湖沼の循環機構に影響を及ぼしたと考えられる．スペクトル解析では2万年周期が検出されるので，この現象は歳差運動に支配されているといえる．しかし，気温上昇よりも珪藻生産の増加が遅れる時期がある．これは気温上昇により形成される土壌起源の栄養塩が，湖内に蓄積する時間を反映する可能性がある．土壌起源の栄養塩を運搬するのは河川なので，土壌形成を促す気温のみならず，降水量も影響しているのであろう．しかし，高野盆地のような閉鎖的で集水域の小さな場所にできた湖沼では，降水量の影響よりも気温に影響を受けることが指摘できる．%A 54-m-long lacustrine sediment core was taken from the Late Pleistocene Takano Formation, Nagano City, Japan, in 2004. The cored sediment consists mainly of homogenous clayey silt associated with many tephra beds, and its major part is the object of this study, covering from 140 to 40 ka in age. The variability of diatom productivity was reconstructed by chemical compositions. The SiO_2/Al_2O_3 ratio shows a correlation with biogenic silica content measured in a part of the same cored sediment. This fact indicates that the SiO_2/Al_2O_3 ratio can be regarded as a proxy for past diatom productivity. The profile of the SiO_2/Al_2O_3 ratio shows a greater similarity to that of pollen-based temperature than to that of pollen-based humidity. This fact implies that the diatom productivity has been mainly controlled by temperature change in this region. When the lake water overturns, stored nutrients in hypolimnion are transported to the photic zone, and support high diatom productivity in general. Thus, the air temperature might control surface water temperature, and subsequently afect the strength and duration of thermal stratification in the lake. This phenomenon must be controlled by a precession cycle, because spectral analysis of the temporal change in the SiO_2/Al_2O_3 ratio revealed a dominant frequency of 20.3kyr. The period of high diatom productivity often delays a temperature rise ; however, this fact may reflect the accumulation time of nutrients from soil produced by temperature rise. Nutrient supply must depend not only on temperature but also on rainfall, because rivers transport nutrients from the soil. In the case of the Takano basin with its small catchment, diatom productivity appears to be influenced by temperature as a more critical factor than rainfall.